A first solution to meet this objective in known in the prior art. European patent EP 140137 describes a system comprising an on-board emitter-receiver unit and a portable emitter-receiver unit. This document describes a solution for recognising the situation in which the badge has been forgotten in the vehicle, by measuring over a determined period the level of the signal received by the on-board receiver, and emitted by the portable device. When the signal variations inside a given period of time are less than a predetermined value, the on-board system produces an alarm signal.
This solution is not entirely satisfactory, as it involves the emission of a signal by the emitter of the on-board device over a relatively long period, of several seconds. As a result there is a significant energy consumption, and this involves the use of a powerful source of electrical energy which is at odds with the aim of miniaturisation and weight reduction. Furthermore, such a solution is highly dependent on the stability of the emission level and may be disturbed by the variations caused by drops in the supply voltage e.g. when the batteries of the portable device are worn out.
Finally, in the case in which several badges remain in the power range, each badge has to be controlled sequentially, which greatly complicates the system.
A second solution is described in European Patent Application EP 426114, which also consists of determining the position of a portable device in the form of a card by means of a detector integrated in the on-board system. The determination of the position is performed not by measuring the variation of the level, but by the use of several antennae installed at various points of the vehicle, and enabling the origin of the emission to be determined, and therefore the position of the card with respect to the interior of the vehicle.
This solution also involves the quasi-continuous emission of a signal from the portable emitter, and therefore has substantially the same drawbacks as the first solution.
The object of the present invention is to propose a system which detects that the portable device has been forgotten, without recourse to a prolonged emission from the portable device, in order to reduce consumption, and to avoid disturbances caused by variations in the supply voltage. Furthermore, a system according to the invention is compatible with a supply of the portable device by induction.